Hello,
I recently came across another fix: Problem: Dovecot UserDB Cache Causing Incorrect Home Directory on Cached IMAP Logins
So far, this seems to have resolved the problem for our customer.
For our own mail server, we’ll need to wait a bit longer, as we don’t have many users and it may take some time before anyone notices if the issue persists.
By the way, our mailserver is also on 5.2 already. My mistake 
Best regards,
Oliver